Why do they move large amount of salt water when they could move only the sweet water? When the plant is 100 meter down, the water pressure is already the requi

Why do they move large amount of salt water when they could move only the sweet water? When the plant is 100 meter down, the water pressure is already the required 10 atmospheres, and when it is offshore at an ocean current the saltifying effect is a not problem. The energy needed is basically the same, but there is much less friction in the pipelines when you move only the non-pressurized end product. In my hand-cranked reverse osmosis plant the difference is 100-fold and you get good 30 minutes exercise for a cup of drinking water. If you live in a submarine, you could just turn the tap and the clean water flows in without any cranking.